1. Field of the Invention
The present invention relates to high-voltage generating circuits, and more particularly, to a high-voltage generating circuit for generating a high voltage applied to the anode of a cathode-ray tube (CRT).
2. Description of the Related Art
A high-voltage generating circuit shown in FIG. 10 is conventionally known. This high-voltage generating circuit 141 roughly includes a high-voltage production circuit 160, a high-voltage detecting circuit 161, and a control circuit 162. The high-voltage detecting circuit 161 detects a fluctuation of a high voltage output from the high-voltage production circuit 160. The control circuit 162 controls a variable DC power source 146 according to a detection voltage output from the high-voltage detecting circuit 161 to compensate the high voltage output from the high-voltage production circuit 160 for its fluctuation and to make the high voltage stable.
The high-voltage production circuit 160 is formed of a switching device 142, a damping diode 143, a resonant capacitor 144, a flyback transformer 145, the variable driving power source 146, a smoothing capacitor 158, and rectifying diodes 156 and 157.
The high-voltage detecting circuit 161 includes voltage-dividing resistors 147, 148, 149, and 150 for dividing the high voltage output from the high-voltage production circuit 160, and speed-up capacitors 151 and 152 for increasing the rise speed of the high voltage output from the high-voltage production circuit 160. voltage-dividing resistors 147, 148, and 149 are connected in series, and together with the speed-up capacitor 151 form a parallel circuit serving as a high-voltage circuit section 161a. The voltage-dividing resistor 150 and the speed-up capacitor 152 form a parallel circuit serving as a low-voltage circuit section 161b. The voltage-dividing resistor 148 also serves as a focus resistor, and the voltage-dividing resistor 149 also serves as a screen resistor. There are also shown a focus capacitor 154 and a screen capacitor 155. The high-voltage generating circuit 141 is of a so-called +B control type. The control circuit 162 controls the voltage of the power source, and the voltage is smoothed by the smoothing capacitor 158 and supplied to the flyback transformer 145.
A cathode-ray tube (CRT) generally has a capacitance of several hundreds to several thousands of picofarads. When a screen A of the cathode-ray tube has a white portion W having a high luminance level (hatched portion B is black) as shown in FIG. 11, a large beam current flows at the white portion W in the cathode-ray tube and a high voltage cannot quickly be generated if the high voltage drops. As a result, the contour of the white portion W on the screen is distorted as shown by dotted lines, which is called a trapezoidal distortion, and the image quality deteriorates.
It has been proposed as a countermeasure to use a capacitor having a capacitance of as large as about 1500 to 3000 pF for the speed-up capacitor 151 of the high-voltage generating circuit 141. However, in that case, the rise speed of the high voltage decreases and it is necessary to extend the blanking period so as not to clearly show a change on the screen until the high voltage rises. Alternatively, a smoothing capacitor having a large capacitance may be provided in addition to the speed-up capacitor 151. In this case, since a high voltage of 1 kV to several tens of kilovolts output from the high-voltage production circuit 160 is applied to this smoothing capacitor, it must satisfy a high dielectric voltage specification and is expensive and large. Therefore, the high-voltage generating circuit becomes large and expensive.